Multi-function monitoring module for a printer

ABSTRACT

In a module for monitoring the print quality of a dot printer ( 10 ), comprising a radiation source ( 41 ), a condenser lens system ( 43 ′) for forming an illuminated area ( 45 ) on a printing medium ( 13 ) and an imaging lens system ( 87,89 ) for imaging the illuminated area on a detector ( 49 ′), the detector comprises at least a linear array of detector elements and the imaging system is configured in such a way that both the illuminated area and a small portion thereof can be imaged. This allows detection of both the gray scale of a print line ( 35 ) and the size and colour of an individual dot. A dot printer provided with this modules gives an improved print quality.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 09/668,540 filed Sep. 25,2000 now U.S. Pat. No. 6, 419, 342.

The present invention relates to a module for monitoring the printquality of a dot printer, which monitor is to be fixed to a carriage formoving a printing device of the printer, said module comprising:

an illumination system for supplying a monitoring radiation beam toilluminate an area of a printing medium;

an imaging system for collecting monitoring radiation reflected by theilluminated area and forming an image in an image plane, and

a radiation-sensitive detection system for converting said image intoelectric signals representing quality parameters of the print.

The invention also relates to a dot printer provided with such amonitoring module.

A dot printer is to be understood to be any kind of apparatus forwriting an image on a recording medium whereby the medium producesmarks, in the form of dots when exposed, for example, to energies abovea threshold level. A number of such dots jointly constitutes acharacter, a sign, a graphic representation, etc. Well-known dotprinters are laser printers, inkjet printers and thermal printers.

U.S. Pat. No. 5,633,672 discloses such an apparatus which comprises acommunication channel for carrying an information signal representingthe image to be written, a writing assembly, or writing device, coupledto said channel and modulated by the information signal for makingmarks, representing the image, on a printing medium in response to thesignal, a device for producing a relative movement in one directionbetween the printing medium and said writing assembly to establish apattern of marks in the direction of the relative movement.

The apparatus described in U.S. Pat. No. 5,633,672 further comprises acalibration instrument that reads the pattern of marks and adjusts theparameters of the apparatus in real time to calibrate the writing deviceand, when necessary, modify the characteristics of the marks and theirpattern. The calibration instrument can be fixed to the drive, orcarriage, for the writing device so that it is moved simultaneously withthis device. By using this built-in calibration instrument, it is nolonger necessary to remove printed images from the apparatus and analyzethem off-line for the characterization of the marks and their pattern,which is a cumbersome and time-consuming process. The controlled writingparameters are the size, the relative position and the density of thedots. In the printer described in U.S. Pat. No. 5,633,672, thecalibration instrument comprises an electronic (CCD) camera having anoptical axis, and the marks are illuminated co-axially along the sameaxis. The camera receives a two-dimensional image of the recorded dotsand their pattern, for example, if the recording medium carrier is adrum at each rotation of this drum, so that all dots of a print line, orof a number of print lines if the writing device is a multi-channeldevice, are taken in simultaneously. The camera reads the spatial anddensity information from the pattern.

It is an object of the present invention to provide a monitoring moduleof the kind described in the preamble, which module allows measurementson a dot-by-dot basis and, moreover, has a simple and compactconstruction. The module is characterized in that the detection systemcomprises at least a linear array of discrete detector elements, and inthat means are provided for imaging, in the plane of the detectorelements, both a medium area of a first kind, comprising a number ofprinted dots, and a medium area of a second kind, comprising one printeddot.

By means of this module it is not only possible to image a portion of aprint line on the detection system and observe it, to monitor thedensity or gray scale of the print line portion and to detect the edgesof the recording medium, or print paper, but it is also possible toimage an individual dot on the detection system. The detection systemhas detector elements which are so small relative to the image of a dotthat such an image covers several detector elements, or pixels, so thatcharacteristics of an individual dot, like its size, can be measured.

The monitoring and calibration can be carried out during a test beforerecording the desired image, or concurrently with image recording.

It is to be noted that EP Patent 0.186.651 discloses a thermal printerwherein individual dot areas are monitored, however during dot formationand not after the dot formation has been completed. The printerdescribed in EP Patent 0.186.651 is used to carry out a special methodof thermal printing. First, initial energy is applied to selected areasof the recording medium to form, in each area, a dot having an initialsize which is smaller than necessary to achieve the desired density inthe selected area. This area is monitored against a reflectivebackground, in the form of a reflective layer on the rear side of thetransparent record medium, by illuminating the area and capturingradiation reflected by the area by a single detector element. Thiselement supplies a signal which is indicative of the density of thearea. This signal is compared with a desired image signal and the resultof the comparison is used to regulate further supply of thermal energyto progressively increase the dot size in the area until a predeterminedvalue of density is achieved. In the thermal printer described in EPPatent 0.186.651, a dot is not monitored by a number of detectorelements and no image of a portion of a print line is formed on thedetection system.

A first embodiment of the module is characterized in that said means forimaging comprises an actuator for positioning the imaging system ineither one of two fixed positions along the axis of said imaging system.

In this embodiment, the same lens element(s) of the imaging system is(are) used to image both an individual dot and a print line portion onthe detection system. When the imaging system occupies a first one ofsaid positions, a dot is sharply imaged on the detection system and whenthe imaging system occupies the second position, a portion of the printline is imaged on the detection system. The detection system thenreceives either an image of an individual dot or an image of the printline portion.

It is also possible to image the individual dot and the print lineportion simultaneously on the same detection system. This can berealized with an embodiment of the module which is characterized in thatsaid means for imaging comprises a first and a second sub-lens system ofthe imaging system, the first sub-lens system having such a power andposition that it image a medium area of the first kind on a first regionof the detection system, and the second sub-lens system having such apower and position that it image a medium area of the second kind on asecond region of the detection system.

Each sub-lens system can now be optimally adapted to the object it hasto image and no movement of the imaging system is required. The imagesof the individual dot and of the print line portion can then beevaluated simultaneously.

Especially for monitoring an individual dot, it is essential that therecording medium, or print paper, remains in the focal plane of theimaging system or of the sub-lens system which images the dot. Inpractice, it has been found that, during the printing procedure, themedium, at the position of the module, may wobble over a range of +0,5mm to −0,5 mm relative to the nominal plane of said medium. A lenssystem with which a dot can be measured at an accuracy of ±10 μm andfits in the small module, i.e. having a sufficiently smallobject-to-image distance, has a depth of focus which is smaller thansaid wobble range. An embodiment of the module which provides a solutionto the important problem of the wobbling medium is characterized in thatit is provided with an auto-focus system comprising a focus errordetection system coupled to a focus correcting actuator.

The auto-focus system ensures that optics the medium is in focus in thefield of view of the imaging.

The embodiment of the module wherein the selection between imaging anindividual dot or imaging a print line portion is made by moving theimaging system may be further characterized in that the focus-correctingactuator is an actuator for fine-positioning the imaging along its axis.

In this embodiment, also a signal from the focus error detection systemmay be supplied to the actuator for selecting the imaging mode, so thatthis actuator performs two functions: selecting the imaging mode, i.e.imaging an individual dot or imaging a print line portion, and tuningthe focus.

Alternatively, the module may be further characterized in that thefocus-correcting actuator is an actuator for positioning a holder,accommodating all optical elements of the module, relative to the mediumin a direction perpendicular to the medium.

The holder actuator can be used not only with the module wherein theimaging system comprises two fixed sub-lens systems, but also in themodule with an undivided, movable, imaging system, although focus tuningby the lens actuator is preferred for the latter module.

The module with a holder actuator may be further characterized in thatthe actuator is constituted by a motor forming part of the module andhaving a cam wheel which is sustained by a ridge on a back plate, whichplate is to be mounted on the carriage.

Upon rotation of the motor, the motor together with the holder isdisplaced axially. This actuator construction becomes possible becausethe module is compact and has a low mass.

Several methods and devices may be used for detecting the focus error.The different embodiments of the module are preferably characterized inthat the focus error detection system comprises a portion of theradiation-sensitive system and an electronic processing circuit which iscoupled to said portion to process and analyze the signals from thedetector elements of said portion in order to determine for whichposition of the axially movable element the image of a feature of themedium has maximum contrast and/or minimum size.

The module may be used in a monochrome printer, but provides an evengreater advantage when used in a colour printer because it then enablesthe colour of an individual dot to be controlled. The module for thispurpose is characterized in that the radiation-sensitive detectionsystem comprises a colour beam splitter for splitting radiation incidentthereon into radiation of different colours and further comprises acorresponding number of sets of detector elements, each set beingintended to receive radiation of one of said different coloursradiation's.

An alternative embodiment of the module for the same purpose ischaracterized in that a separate colour filter is arranged in front ofeach detector element of the radiation-sensitive detection system, thecolour filters for neighboring detector elements transmitting radiationof different colours.

No separate colour separating element is needed in this embodiment ofthe module.

If only the presence of the different print colour component: cyan,magenta, yellow and black in the print needs to be controlled, i.e. ifit is only to be checked whether the nozzles for the different inkcolours indeed eject ink, the radiation source of the illuminationsystem needs to emit radiation of one wavelength band, provided thatradiation of this wavelength is sufficiently absorbed by the printedcolour dots. It has been found that a light-emitting diode (LED) havinga central wavelength of 430 nm is suitable for this purpose. If not onlythe presence of print colour components, but also the colour compositionof printed dots, i.e. the relative amount of print colour components, isto be controlled, a radiation source should be used that suppliesradiation comprising colour components which are absorbed most by thecyan, magenta, yellow and black components of the dot. Such a source maybe a single source, for example, a small lamp which emits white light.

Preferably, however, the module is further characterized in that theillumination system comprises at least one set of differentlight-emitting diodes each emitting radiation of a different colour.

Light-emitting diodes, or LEDs have proved to be reliable radiationsources which may be small and cheap and are available for differentcolours. In practice, the radiation source of the module will comprise aset of three different LEDs emitting the primary colours red, green andblue. Under circumstances, for example, when a higher intensity for themonitoring radiation is required and sufficient room is available, twoor more sets of such LEDs may be used.

The LEDs can be switched on simultaneously and the different printcolour components are then detected simultaneously. It is also possibleto switch on the LEDs successively and to detect the print colourcomponents successively. In the latter case the same detector elementscan be used to detect the different print colour components, so thatfewer detector elements are needed than in the case where these colourcomponents are detected simultaneously.

If the solid angle of the radiation emitted by the radiation source isnot too large, the radiation source may directly illuminate the printingmedium. For example, the LEDs may be provided with lens elements whichconstrict the emitted radiation.

Preferably, however, the module is characterized in that theillumination system comprises a convergent lens system.

Such a lens system concentrates a maximum amount of the source radiationon a limited area of the printing medium so that the source radiation isused efficiently.

The module is preferably further characterized in that the illuminationsystem comprises a cylindrical lens to form an elongated illuminationspot on the medium, the longitudinal direction of the spot beingparallel to the direction of a print line.

Such a shape and orientation of the illumination spot fits best for thepurpose of the module.

The invention also relates to a dot printer for writing an image on arecording medium by exposing the medium on a dot-by-dot basis andcomprising:

-   -   a communication channel for carrying an information signal        representing the image to be printed;    -   at least one writing device coupled to the communication channel        and modulated by the information signal for producing dots on        the recording medium, together composing the image;    -   a carriage for carrying the writing device for producing a        relative movement between the recording medium and the writing        device, and    -   a calibration instrument, fixed to the carriage, to read the        printed dots and adjust printer settings. This dot printer is        characterized in that the calibration instrument comprises a        module as described hereinbefore.

The invention further relates to a method of printing information in theform of dots on a printing medium, comprising the steps of:

-   -   supplying the information to be printed to a dot printer;    -   printing a test pattern on the medium;    -   optically detecting the test pattern;    -   comparing the test pattern with a reference;    -   adapting printer settings if a deviation between the reference        and the test pattern is detected, and    -   printing the information supplied to the printer.

This method is characterized in that the step of detecting the testpattern comprises detecting the gray scale of a test print line anddetecting the size of at least one individual test dot.

This method ensures a high print quality, also with consumer printers.

This method is preferably characterized in that the step of detectingthe test pattern also comprises detecting the colour of a test dot.

The method may be further characterized in that the step of printing atest pattern comprises printing a first portion of the information to beprinted.

If dots and print line portions of the print image itself are used fortesting, the time required for the calibration process is minimum.

These and other aspects of the invention are apparent from and will beelucidated, by way of non-limitative example, with reference to theembodiments described hereinafter.

In the drawings:

FIG. 1 shows a view of an embodiment of an inkjet printer;

FIG. 2 shows a carriage for the cartridge for this printer provided witha monitoring module;

FIG. 3 shows a portion of a printing medium provided with print linesand the footprint of the cartridge and the module;

FIGS. 4 a and 4 b show a first embodiment of the monitoring module setin a dot monitoring mode and in a print line monitoring mode,respectively;

FIG. 5 shows an embodiment of a convergent lens system of this module;

FIG. 6 shows a part of an embodiment of the module for colour detection;

FIG. 7 shows a linear detector array provided with colour filters forthis module;

FIGS. 8 a and 8 b show a first and a second cross-section of a secondembodiment of the module;

FIG. 9 shows a first embodiment of a focus error detection system foruse with the module;

FIG. 10 shows a second embodiment of the focus error detection system;

FIG. 11 shows a third embodiment of the focus error detection system;

FIG. 12 shows a front view of the detector in this embodiment, and

FIGS. 13 a and 13 b illustrate a preferred embodiment of a focus errordetection method.

FIG. 1 shows an embodiment of an inkjet printer 10, known from EP-A0,641.115. This printer has an internal frame which includes atransverse rod 11, extending in an Y direction. The internal framedefines a print media path 12 through which the print media passes inthe X direction. A sheet of print paper 13 is shown passing through theprint media path suspended on paper rails 14 and 15. The means formoving the paper through the print media path are well known and notshown in FIG. 1. The internal frame is encased by an enclosure 17.

Mounted on the internal frame is a carriage 19 which houses a writingdevice, or inkjet cartridge, 21. The means for moving the carriage 19 inthe Y-direction are not shown. These means are also well-known in theart. Usually, the print paper in an inkjet printer is displaced in the Xdirection whenever the carriage has finished one pass over the paper inthe Y direction. The extent of displacement of the print paper isdetermined by the x dimension of the area covered by the cartridge 21.The inkjet cartridge contains a reservoir of ink and a print headcontaining the ink-jetting mechanism. The print head comprises a numberof individual nozzles arranged, for example, in two or three columns,through which the ink is ejected onto the print paper area. Thecartridge also comprises a flexible circuit interconnection throughwhich the printer electronics (not shown) selectively activates eachnozzle so that this nozzle ejects a drop of ink.

Each nozzle will only eject ink when a drop is needed at apre-determined position on the print paper. The Y position of the printhead with respect to the paper is controlled by the cartridge transportmechanism. To determine the position of the cartridge, and thus of theprint head, the carriage is provided with a position sensor, forexample, an optical sensor that moves along a scaled strip of theinternal frame.

The nozzles can eject several ink drops on approximately the sameposition. These drops blend together into an ink strain, called a dot.The dot is considered as the smallest print unit. The size of such a dotdepends on the amount of ejected ink. If all nozzles eject black ink, ablack dot is formed on the print paper. To obtain a colour dot, the inkhead should contain nozzles of different kinds whereby each kind ejectsa different ink colour. By blending drops of different colours, a dothaving one specific colour from a large range of colours is obtained.The colour of such a dot is determined by the number of drops of eachejected colour. In practice, a sufficiently large gamma of colours isobtained with three ink colours cyan, magenta, yellow, and with black.The printer may comprise one cartidge for printing in black and onecartridge for colour printing, in which can both cartridges are mountedon the same carriage. The printer may also be provided with only onecartridge which comprises both black ejecting nozzles and colourejecting nozzles, so that both black prints and colour prints may beproduced by the same cartridge. Alternatively, four cartridges, ejectingblack, cyan, magenta and yellow ink, respectively, may be used. This maybe advantageous when it is expected that more ink of one or more colourswill be used than ink of the other colours.

There is a need to measure the quality of prints produced by such inkjetprinters and by printers of another kind such as laser printers orthermal printers. Such a measurement can be performed during a testprinting procedure and before recording the desired image, so that thewriting device can be calibrated using the results of the measurement ofthe test print. Preferably, however, the measurements are also carriedout during printing features, which enables deviations to be detectedimmediately after the features have been printed, so that, for example,printing can be stopped at an early stage when such deviations occur andink, time and paper can be saved.

Said measurement can be carried out by an optical module having suchsmall dimensions and mass that it can be fixed to the carriage for thecartridge so that it can be moved together with said carriage. FIG. 2shows a front view of the carriage 19 with the monitoring module 23. Thecarriage may be designed to accommodate either one cartridge or, asshown in FIG. 2, two cartridges 21,21′. The module may be arranged insuch a way that it trails the cartridge(s), thus at the left side of thecartridge(s) if, during printing, the carriage moves to the right. Thiswill be the case if monitoring is carried out continuously andimmediately after printing or if, after printing a line, the carriagehas been moved back to the left, monitoring is carried out during thenext movement to the right. If monitoring is carried out after a linehas been printed and during the backward movement of the carriage to theleft, the module is arranged at the right side of the cartridge(s). Themodule is at the same level of the printing medium, or paper, as thecartridge(s). Thus the monitor can observe printed features duringprinting of a print line or after such a line has been printed.

FIG. 3 shows a footprint 33 of the cartridges and monitoring module,i.e. the projection of these elements, on a portion of the print paper13 and two print lines 35 and 36 on this paper. A print line is thestroke of the printing medium that is scanned and printed by the inkhead in one go, from left to right. In practice, such a print line has amaximum width of, for example 12.5 mm. The module comprises a radiationsource and a first, condenser, lens system by means of which an area ofa print line is illuminated. This area is imaged by a second, imaging,lens system on a radiation-sensitive detection system, hereinaftercalled detector. A monitor that illuminates an area which is smallerthan the width of a print line with one colour and comprises a singledetector element has the capabilities of detecting the gray scale of aprinted area and detecting the paper edges. By looking at the gray scaleof, or density in, the illuminated area, it can be established whetherone of the cartridges does not eject ink, by detecting whether ink ismissing at a position where it should be. However, this is only possibleif the radiation emitted by the radiation source has such a wavelengththat it is sufficiently absorbed by the ink. By determining the edges ofthe paper, it can be established whether the paper fed into the printerhas the required size and whether this paper is fed through the printerin a correct way. With such a monitor, having a small field of viewrelative to the width of a print line, only a small portion of the printline width can be scanned and only relatively large deviations aredetected.

The present invention provides a monitoring module that has morecapabilities and enables finer measurements to be made. This modulecomprises a radiation-sensitive detection system composed of a largenumber of detector elements and an imaging lens system that enables theilluminated printing medium area, or portions thereof, to be imaged onthe detection system in two different magnifications. In the firstmagnification, a medium area with a dimension of the order of that of adot is imaged on the detection system, whereby the dot image coversseveral of the individual detector elements, so that it becomes possibleto determine the quality of an individual printed spot. In the secondmagnification, the whole illuminated area or a considerable portionthereof is imaged on the detection system, so that also gray scalemeasurement and printing medium edge detection remains possible. Thedetection system may comprise a, for example linear, CCD array.

FIG. 4 a shows a cross-section of a first embodiment of the monitormodule. In this FIG., element 41 is a radiation source, which ispreferably a white source and is most preferably composed of at leastone set of three light-emitting diodes (LEDs) which emit red blue andgreen light, respectively. FIG. 4 a shows only two LEDs 41′, 41″ of aset of three. The radiation source is adapted to the kind of monitoringto be carried out. For example, the number of differently coloured LEDsdepends on the colours to be detected. If the radiation emitted by thesource 41 has a solid angle that is not too large, this source maydirectly illuminate the printing medium 13. When using LEDs forillumination, they may be provided with lens elements which constrictthe emitted radiation. Preferably, the monitor comprises a converginglens 43 which converges the light from the source 41 on the printingmedium so that only an area 45 having a pre-determined size of themedium 13 is illuminated. The converging lens also prevents to a certaindegree that radiation which does not come from the print area to beobserved reaches the detection system 49. Element 47 is an imaging lenssystem which images the illuminated area on the detection system 49.This system is, for example a linear CCD array. The imaging lens systemis, for example, a refractive lens having either two aspherical surfacesor one or two aspherical surfaces. The lens material may be glass, butis preferably made of a transparent plastics material, like PMMA(polymethyl methacrylate) or PC (polycarbonate). Such a lens element canbe mass-produced at low cost by means of well-known replicationtechniques. The imaging lens may also be a Fresnel lens or a diffractiveelement, which lens and element can also be mass-produced at low cost.

To ascertain that the illuminated area is in the field of view of thedetection path, comprising the imaging lens 47 and the detection system49, the radiation from the source should not only be converged, but alsobend toward the left. To that end, an optical wedge could be inserted inthe illumination path from the radiation source to the printing medium.The lens 43 may be a refractive lens having two spherical surfaces, onespherical and one aspherical surface or two aspherical surfaces. Thelens 43 may also be a Fresnel lens or a diffractive element. Also in anembodiment wherein the radiation source directly illuminates theprinting medium an optical wedge may be inserted between the radiationsource and the printing medium for binding the source radiation towardsthe print area to be observed.

FIG. 5 shows a preferred embodiment of the first lens system 43. Itconsists of one element having a wedge-shaped upper side 43 a and aFresnel structure 43 b at its underside. Also this lens element may bemade of glass, but preferably of PMMA or PC.

The multi-element detector and the LEDs are preferably mounted on acommon printed circuit board (PCB) 51 which comprises circuits forsupplying and switching the LEDs and for reading out the detectorelements and transporting the information signals to a centralprocessing unit of the printer. In this unit, the signals from thedetector are compared with reference signals, and control signals aregenerated for the printer settings.

A cylindrical lens is having its cylindrical axis in the X direction isarranged in front of the LEDs, or another radiation source, so that anelongated area of the printing medium is illuminated whereby thelongitudinal direction of the area is in the transversal direction of aprint line. The illumination path is designed in such a way that theilluminated area has a dimension of the order of 12.5×0.085 mm, whichmeans that the illuminated area covers the whole width of a print lineover a length of the order of one dot size. By scanning this illuminatedarea over the medium by means of the carriage 19, a full print isscanned and successively imaged on the composed detector 49.

In the embodiment of FIGS. 4 a and 4 b, the selection between the modesof imaging the whole illuminated area and imaging only a portion havinga size of the order of that of a dot is made by positioning the singleimaging lens 47 in one of two axially different positions. This lens isaccommodated in a holder 53 which can be moved by well-known actuator,or motor, means. In the mode depicted in FIG. 4 a, the lens 47 is in afirst, low, axial position close to the printing medium 13. In thisposition, the lens 47 observes only an area of the order of a single dotand images this area on the composed detector 49 with a high resolution.The resolution is the ability of a lens system to reproduce points,lines etc. in an object as separate entities in an image. The resolutionin the first mode of FIG. 4 a is considerably larger than is needed todiscriminate between individual dots, so that the size and shape of anindividual dot can be determined in this mode.

In the second mode illustrated in FIG. 4 b, the lens 47 is in a second,high, position, most remote from the printing medium. In this position,the lens observes and images the whole print line width on the composeddetector with a resolution that is lower and such that individual dotscannot be resolved. However, this mode is very suitable for measuringthe intensity, or gray scale of a print line and the left and right edgeof the printing medium.

In order to prevent that radiation from the radiation source, notreflected at the illuminated area, reaches the detector, a radiationshield 55 may be provided in the module, as shown in FIGS. 4 a, 4 b.

In the above-mentioned first mode, not only the size and shape of anindividual dot, but also the colour of such a dot can be determined. Forspot size and spot shape determination, a radiation source emittingradiation of a single colour or colour band will do. However, fordetermining the colour, a radiation source should be used which emitsradiation having a colour spectrum that covers the printer colours cyan,magenta, yellow and black. Such radiation is, for example, emitted by awhite light source. Such a white light source is preferably composed ofa red, green and blue LED. These LEDs may be switched on simultaneouslyor sequentially.

When all LEDs are activated simultaneously, the printing medium isilluminated with white light and simultaneous colour scanning becomespossible. A colour-separating element, such as a prism or a diffractiongrating, is arranged in the detection path to divide the white lightbeam reflected from the illuminated area into three differently colouredsub-beams. These sub-beams have such different directions of propagationthat they are incident on different elements of the composed detector49. Each object pixel is then imaged on three detector elements. Thecolour-separating element is preferably arranged between the imaginglens 47 and the detector 49. Three additional lenses may be arranged infront of this detector to direct and concentrate the sub-beams withdifferent colours on their associated detector elements.

This is illustrated in FIG. 6, which shows only the portion of thedetection path between the imaging lens 47 and the detection system 49.The white imaging beam 61, which is focused by the imaging lens 47 inthe plane of the detection system, passes through a diffraction grating62. This grating diffracts the red, green and blue components of thewhite imaging beam 61 through different diffraction angles, so that thewhite beam 61 is divided into three differently coloured sub-beams 63,64 and 65, the chief rays of which are incident on different detectorelements 66, 67 and 68, respectively. Also the other corresponding raysof the sub-beams 63, 64 and 65 are incident on different detectorelements. To improve the imaging quality of the module an additionallens may be arranged between the diffraction grating 62 and thedetection system 49.

The simultaneous colour scanning embodiment of the module enables thecolour of the dots of a print line to be determined in one scan of themodule over this print line.

An alternative to the simultaneous colour scanning embodiment is thesequential colour scanning embodiment. In the latter embodiment, theprinting medium is successively illuminated by red, green and blue lightby activating the red, green and blue LEDs, respectively. Theillumination beams of the three LEDs are now successively projected onthe same illumination area. The reflected light beams with differentcolours are now successively projected on the same detector elements, sothat the number of detector elements needs to be only ⅓ of the number ofdetector elements in the simultaneous colour scanning embodiment. Todetermine the colour of the dots of a print line, this line has to bescanned three times. By using a suitable detection system and suitableLEDs, a very acceptable total scanning time can be obtained. Comparedwith the simultaneous colour-scanning embodiment, the sequentialcolour-canning embodiment has the advantages that it has a simplerconstruction and that no measures have to be taken to prevent the lightsof the different colours from being mixed.

For imaging the small object pixels, i.e. portions of a dot, having asize of, for example 10×10 μm, the imaging lens should have a highresolution. The resolution is proportional to the numerical aperture,NA, of the lens. As the depth of focus of the lens is inverselyproportional to NA², a lens with a high resolution has a small depth offocus. An object can be imaged sharply only if it remains within thedepth of focus of the imaging lens. However, in practice, the level ofthe printing medium at the location of the monitoring module can varybetween +0,5 mm and −0,5 mm relative to the nominal level, if noadditional measures are taken. This range is larger than the depth offocus of the imaging lens, which means that object pixels of saiddimension can no longer be resolved, i.e. imaged separately. In order toavoid this, the module is preferably provided with an auto-focus systemensuring that the optimum distance between the imaging lens and theprinting medium is continuously maintained.

Although it is not a must, the auto focus system is an important noveland inventive aspect of the present invention, because it provides avery attractive solution to a new problem.

The auto-focus system comprises a focus error detection system fordetecting a deviation between the object plane of the imaging lens 47and the plane of the printing medium 13 and correcting means coupled tothe focus error system to such a deviation or eliminate reduce it to anacceptable level.

For the embodiment of FIGS. 4 a and 4 b. the most attractive way ofeliminating or reducing the deviation is to displace the detector 49 orthe imaging lens 47 axially. Displacement of the lens is preferredbecause it is not connected to other elements. The actuator needed torealize the displacement may be, for example, a piezo-electric actuatoror a motor. A motor is preferred because it is cheaper than apiezo-electric actuator and still meets the requirements with respect toaccuracy of movement and response time. The motor may be a linear motoror a rotational motor. A linear motor has the advantages that it drivesthe object to be moved with high precision at large velocities and largeaccelerations and has a high reliability and a long lifetime. Arotational motor requires a transmission to convert the rotationalmovement into a linear movement. However, this motor is more compactthan a linear motor and, because it is a standard product, it is cheaperthan a linear motor. The latter aspect is important for a mass-, orconsumer, product, which the printer and thus the module is intended tobe. As, moreover, a rotational motor has a sufficient performance forthe present job, it is preferred to use this kind of motor. There areseveral options to convert the rotational movement of the motor into alinear movement of the lens. For example, the motor may be provided witha cog, an eccentric wheel, a cam wheel or a worm gear or use may be madeof friction between a round motor wheel and the object to be displaced.

With respect to the auto-focus aspect, the embodiment of FIGS. 4 a and 4b has the advantage that, for correcting focus errors, only the smalland light lens element 47 needs to be moved, which allows a fastresponse to focus errors and a continuous correct setting of the module.Moreover, the same actuator, or motor, can be used for moving theimaging lens 47 between the first axial position (FIG. 4 a) as well asthe second axial position (FIG. 4 b) and for correcting focus errors.

FIG. 8 a shows a first cross-section of a second embodiment of themonitoring module. This module comprises the same radiation source 41,preferably three LEDs and a similar detection system 47′ and a commonPCB 51 for the LEDs and the detector. This module has the capability toimage both a dot and the width of a print line simultaneously onseparate portions of the composed detector. The imaging system isdivided into two subsystems, each comprising a lens 87 and 89,respectively. These lenses have fixed positions and differentmagnifications. The lens 87 performs a similar function as lens 47 inthe mode of FIG. 4 a, namely imaging, by means of beam 91, a printingmedium area with a size of the order of one dot on the left portion ofthe detection system. The lens 87 and its holder 88 are fixed to a sidewall of the module enclosure 81. The lens 89, also at a fixed position,performs a similar function as lens 47 in the mode of FIG. 4 b. Itimages, by means of beam 92, a medium area having a dimension of theorder of the width of a print line in one direction and a dimension ofthe order of one dot in the other direction on the right portion of thedetection system 47′. It should be ensured that the beam 92 is incidenton the detection system 47′ at another position than the beam 91. Tothat end, the lens 89 may have a wedge shape so that it deflects thebeam 92 away from the beam 91.

The illuminated area 45 may be the same as that in FIG. 4 a and thecondenser lens 43′ may have the same lens power and deflection power asthe lens 43 in FIG. 4 a. As shown in FIG. 8 a, the lens 43′ may have awedge structure. The lens 43′ may also be a normal spherical oraspherical lens which is arranged in such a way that the illuminationbeam 90 passes excentrically through this lens, so that the beam isdeflected. This also holds for the condensor lens 43 in the embodimentof FIG. 4 a. The imaging lens 89 is preferably designed in such a waythat it can be arranged at the same Z position as the condensor lens43′, so that these two lenses can be integrated into one lens element,as shown in FIG. 8 a. This lens body, or the separate lenses 89 and 43′may be made of PMMA or PC and can be mass-produced at low cost by meansof well-known replication techniques. This also holds for the lens 87,which may also be made of PMMA or PC.

Also in the embodiment of FIG. 8 a, a light shield 55′ may be providedto prevent radiation from the source 41 and not reflected at theilluminated area 45 from reaching the detector 49′. Because of thedifferent construction of the module of FIG. 8 a, this light shield hasa shape and dimension which are different from those of the shield 55 inthe module of FIG. 4 a. The module of FIG. 8 a preferably comprises asecond light shield 94 arranged between the paths of the imaging beams91 and 92 to prevent radiation of any of these beams from reaching adetector portion which is intended for the other beam.

The lens 87 images on the detection system only a medium area of theorder of one dot with a resolution of, for example 10 μm. A resolutionof 10 μm means that object details spaced 10 μm apart can be separatelyimaged and detected by an adapted detector. This dimension isconsiderably smaller than the average dot diameter, which is, forexample of the order of 100 μm, and also smaller than the smallest dotdiameter which is, for example of the order of 75 μm. The detectionsystem 49′, preferably a CCD sensor, should be adapted to the requiredresolution and the geometric distances between the medium and the lens87 and between this lens and the detector. Moreover, the module shouldbe as small as possible. Said distances are mainly determined by themagnification M of the lens 87. This magnification is the ratio betweenthe size of the object pixels, i.e. picture elements, and the size ofthe detector pixels, i.e. detector elements. For a practical embodiment,the most suitable distances are obtained for a magnification M=1.3 and asize of 13×13 μm for a detector element. For measuring the size andcolour of a single dot, it is sufficient to image a medium area of300×300 μm. For a resolution of 10 μm, 30 pixels of a linear detectorarray are then needed for this measurement. To measure a print linethroughout its width, a resolution of the order of 80 μm is sufficient.For a print line width of 12.5 mm, the required number of detectorpixels is of the order of 150. Of the available linear CCD arrays, anarray with 256 pixels is most suitable for the present purpose. 150 ofthese pixels, in the centre and at the right side of the array, are usedto measure the print line, and 30 pixels at the left size are used tomeasure a dot. The remaining pixels, about 70 in number, between thearea of the dot measuring pixels and the area of the line measuringpixels form a neutral, i.e. unread, area which prevents cross-talkbetween the measurements.

Auto-focus is carried out with the module of FIG. 8 a by moving thewhole module, thus also the lenses 87 and 89, in the axial, or Z-,direction relative to a back plate which is mounted on the carriage. Therange of this movement is, for example from +0,5 mm to −0,5 mm relativeto the nominal position. This movement can be realized, for example, bya rotational DC motor 95 which has an eccentric, or cam, wheel 97. Aridge 99 of a back plate 101, shown in FIG. 8 b supports this wheel.FIG. 8 b shows a cross-section of the module at the position of themotor and along a plane perpendicular to the plane of cross-section ofFIG. 8 a. To ensure that the cam wheel always contacts the ridge, aspring element 103 which is integrated with the back plate 101 exerts adownward force on the motor 95. If the wheel 97 rotates from its nominalposition in one direction, the motor and the module move upwards and ifthe wheel rotates in the opposite direction, the motor and the modulemove downwards.

For automatic adaptation of the height, or Z, position of the module ofFIG. 8 a, or this position of the imaging lens 47 in the module of FIG.4 a, the motor 95 for the module or the motor for said lens should besupplied with a focus error signal. This signal, which represents adeviation between the plane of focus of the imaging lens and the planeof the printing medium at the location of the module can be obtained inseveral ways.

A first method of generating a focus error signal is the triangulationmethod. FIG. 9 shows the basic device for performing this method. Thisdevice comprises a radiation source 110, additional to the source 41 inFIGS. 4 a and 4 b. The source 110 supplies a focus detection beam 11which is incident on the printing medium 13 at an acute angle and isreflected by the medium as beam 112. A radiation-sensitive detector 114,comprising two detector elements 115 and 116 separated by a strip 117,is arranged in the path of the reflected beam 112. The focus detectiondevice is designed in such a way that, if the printing medium is infocus, as indicated by the solid line 13, a spot formed by the reflectedfocus beam 112 in the plane of the detector 114 is symmetric relative tothe separating strip 117. If the medium drifts from the focal plane, asindicated by the broken line 13′, the reflected beam 112 is displacedparallel to itself, as indicated by the beam 112′. Then the spot formedin the detector plane moves over the detector elements, so that one ofthese elements receives more radiation than the other. By measuring thedifference of the signals from the detector elements, the extent andsign of a focus error can be determined.

A second method of generating a focus error signal is the skew beammethod. FIG. 10 shows the basic device for performing this method. Thisdevice comprises a radiation source 120 which supplies a, for examplecollimated, focus detection beam 121. This beam is incident on theprinting medium 13 at an acute angle and is reflected by the medium asbeam 122 towards a radiation-sensitive detector 125. A lens 123 whichfocuses the beam 122 in the plane of the detector is arranged betweenthe medium 13 and the detector 125. The detector 125 comprises twodetector elements 125 and 125, separated by a strip 127. The device isdesigned in such a way that, if the printing medium is in focus, asindicated by the solid line 13, the beam 122 incident on the detector issymmetric with respect to the separating strip, so that the twodetectors receive the same amount of radiation. If the medium shiftsfrom the focal plane, as indicated by the broken line 13′, the reflectedbeam shifts parallel to itself and the position, from the lens axis,where the chief ray of this beam enters the lens 123 shifts. The chiefray is then refracted by the lens, as indicated by the interrupted ray122′ and is incident on one of the detector elements, so that thiselement receives more radiation than the other detector element. Bymeasuring the difference of the signals from the detector elements, theextent and sign of a focus error can be determined.

Still another focus error detection method is the astigmatic method.FIG. 11 shows the basic device for performing this method. This devicecomprises a radiation source 130 which supplies a, for examplecollimated, focus detection beam 131. This beam passes through afocusing lens 132 which converts the beam 131 into a convergent beam 133and focuses beam 133 into a spot 134 on the information medium 13. Thismedium reflects a part of the beam 133 as reflected beam 135. Arrangedbetween the focusing lens 134 and the medium 13 is a beam splitter, forexample a semi-transparent mirror 137 or a beam-splitting prism, whichtransmits a part of the beam 133 and reflects a part of the beam 135towards a detector 140. The spot 134 on the medium is re-imaged in aspot 139 on the detector 140. An astigmatic element, for example acylindrical lens 138 is arranged in the path of the reflected beam 135.The effect of the astigmatic element is that a point of the medium 13 isno longer imaged in one focal point, but in two astigmatic focal lineswhich are shifted with respect to each other along the axis of the beam135. Upon a shift of the medium 13 in the direction of the axis of thebeam 133, the astigmatic focal lines of the beam shift along the axis ofthe beam 135, and thus also with respect to the plane of the detector140. FIG. 12 shows a front view of the detector 140 with the spot formedon this detector 140. The detector comprises four detector elements 141,142, 143 and 144 and is usually called a quadrant detector. The detector140 is placed between the astigmatic focal lines. The device is designedin such a way that, if the medium is in the focal plane of the lens 132,the plane of the detector is midway between the astigmatic lines, i.e.the distance between one astigmatic line, before the detector plane, isequal to the distance between the other astigmatic line, behind thedetector, and the detector plane. In this situation, the spot formed onthe detector is a round spot and the four detector elements receive thesame amount of radiation. If the printing medium 13 shifts with respectto said nominal position, the spot on the detector deforms to anelliptical spot with the longitudinal direction at +45°, spot 147, or at−45°, spot 148. If spot 147 is formed, the detector elements 142 and 144receive more information than the detector elements 141 and 143. If spot148 is formed, the detector elements 142 and 144 receive less radiationthan the detector elements 141 and 143. By adding the signals from thedetector elements 141 and 143 and the signals from the detector elements142 and 144 and subtracting the obtained sum signals, the extent andsign of a shift of the printing medium 13 with respect to the focalplane of the lens 132 is obtained.

The focus error detection methods described above require additionalelements to be arranged in the module. Together with the detectionsystem only the focus detector elements may be integrated into onecomposed detection system for determining the print quality. Saidadditional elements are not needed if use is made of a focus errordetection method which searches for the smallest dimension and highestcontrast of the image of a dot. For an optical system like the present,wherein the maximum dot size is smaller than 0.3 mm and the lens has adiameter of, for example 3 mm, thus in a system wherein the object issmaller than the lens diameter, the size of the image is minimum and thecontrast is maximum for the in-focus state. If the system gets out offocus, the image will get larger and its contrast lower. This isillustrated in FIG. 13 a which shows the image of a dot on the detectorelements. The right side of this Figure shows the situation wherein theprinting medium is in focus. The dimension of a dot image 150 is thenminimum so that a minimum number of detector elements 155 is covered.The contrast is then maximum. The left side of FIG. 9 a represents thesituation in which the printing medium is out of focus. The size of thedot image has increased and the contrast has decreased, which isillustrated by the lighter ring 151 around the in-focus image 150. Inthe out-of-focus condition, a larger number of detector elements 155 iscovered. By comparing the signals from the detector elements 155, thesize and contrast of the dot image can be determined. The information ofthis measurement is used to control the focus-correcting motor, so thatthe size is maximum and the contrast minimum.

The measurement of the dot image size and contrast can be carried out ona small slice of the image, as shown in FIG. 13 a, provided that theimage is stationary with respect to the detector. This condition becomesnecessary when the dot image has an irregular shape. It is thereforepreferred to measure the whole image, which can be realized by movingthe image and the detector relative to each other as illustrated by thearrow 157 in FIG. 13 b. In FIGS. 13 a and 13 b, the same referencenumerals represent the same elements.

The focus error detection system of FIG. 13 a or 13 b has the advantagethat it can be implemented in the monitoring module without adding anyadditional element to the module. The size of the module with this focuserror detection system is determined only by the design of the opticalsystem for determining the size and colour of a dot, the gray scale of aprint line and the edges of the printing medium. A practical embodimentof the module has a height of 25 mm, a width of 15 mm and a depth of 10mm. This module is very suitable to be mounted on the carriage for theprinting device(s).

Because of the measuring capabilities of the present module, a printercomprising this module has the unique feature of automatic adjustment ofthe printer settings to the characteristics of the printing medium used.A novel, improved print quality can be obtained by means of such aprinter.

As said measurements cannot be carried out simultaneously, they shouldbe done sequentially. A possible sequence for the measurements is asfollows. After the printer has received a print instruction, it willfeed a sheet of paper under the printing device. The monitoring moduleis positioned so as to detect the top of the paper as it passes throughthe printing device. The paper is transported further until it reachesthe position for the first print line. The module checks whether theright side of the paper is aligned correctly relative to the printer.Then the printer prints several separate test dots, which are part ofthe image to be printed. The carriage moves until the module detects theleft side of the paper. The width of this paper is then known by theprinter. Then the carrier moves back. During this movement, the monitormeasures the location, size and colour of the test dots. For each dotmeasurement, the printer positions the module above the dot and focuseson the dot, so that a sharp image of the dot is obtained. After the testdots have been measured, their average size, colour and positions arecompared with stored reference values. If deviations occur, the settingsof the printer are adapted and stored in the memory of the printer. Inthis way, the printer automatically aligns its cartridges and achievesmaximum print quality for each medium type, medium colour andenvironmental condition. The printer is then ready to print the requiredinformation. The carriage moves back to its vertical start position andthe carriage moves to the left whereby it scans the blank paper that isto be printed and checks whether the print line has not been printedalready. If ink is detected, this could mean that the paper slips or thewrong paper, has been fed in in case of double sided printing. Duringscanning back by the carriage, the module controls a just printed line,and detects whether the dots are at the right position and together showthe right gray scale. This informs the printer about the status of thecartridges. Whenever the module passes a paper edge during the printjob, the position and orientation of the paper is controlled. The modulepossible detects the underside of the paper passing under the printingdevice. The length of the paper measured by means of the module isstored in the memory of the printer. If subsequent sheets of paper seemto be longer, the printer will check whether this is not due to morethan one sheet of paper being fed in.

The invention has been elucidated with reference to an inkjet printer,but this does not mean that it can only be used in combination with sucha printer. The invention can generally be used in any kind of dotprinter which has controllable printer settings and prints opticallydetectable dots and print lines.

1. A module for monitoring the print quality of a dot printer, whichmonitor is to be fixed to a carriage for moving a printer device of theprinter, said module comprising: an illumination system for supplying amonitoring radiation beam to illuminate an area of a printing medium; animaging system for collecting monitoring radiation reflected by theilluminated area and forming an image in an image plane, and aradiation-sensitive detection system for converting said image intoelectric signals representing quality parameters of the print,characterized in that the detection system comprises at least a lineararray of discrete detector elements, and in that means are provided forimaging, in the plane of the detector elements, both a medium area of afirst kind, comprising a number of printed dots, and a medium area of asecond kind, comprising one printed dot.
 2. A module as claimed in claim1, characterized in that said means for imaging comprised an actuatorfor positioning the imaging system in either one of two fixed positionsalong the axis of said imaging system.
 3. A module as claimed in claim1, characterized in that said means for imaging comprises a first and asecond sub-lens system of the imaging system, the first sub-lens systemhaving such a power and positioned that it images a medium area of thefirst kind on a first region of the detection system, and the secondsub-lens system having such a power and positioned that it images amedium area of the second kind on a second region of the detectionsystem.
 4. A module as claimed in claim 1, characterized in that it isprovided with an auto-focus system comprising a focus error detectionsystem coupled to a focus correcting actuator.
 5. A module as claimed inclaim 4, characterized in that the focus-correcting actuator is anactuator for fine-positioning the imaging system along its axis.
 6. Amodule as claimed in claim 4, characterized in that the focus-correctingactuator is an actuator for positioning a holder, accommodating alloptical elements of the module, relative to the medium in a directionperpendicular to the medium.
 7. A module as claimed in claim 6,characterized in that the actuator is constituted by a motor formingpart of the module and having a cam wheel which is sustained by a ridgeon a back plate, which plate is to be mounted on the carriage.
 8. Amodule as claimed in claim 4, characterized in that the focus errordetection system comprises a radiation source for supplying a focusdetection beam, the chief ray of which is at an acute angle with anormal to the plane of the printing medium, and a radiation-sensitivedetector, comprising two detector elements separated by a strip andarranged in the path of the focus detection beam reflected from themedium so that the cross-section of this beam in the plane of thedetector is symmetrical with respect to the separating strip if theprinting medium is in focus with the imaging system.
 9. A module asclaimed in claim 4, characterized in that the focus error detectionsystem comprises a radiation source for supplying a focus detectionbeam, the chief ray of which is at an acute angle with a normal to theplane of a printing medium, and, successively arranged in the path ofthe focus detection beam reflected from the medium, a lens system and aradiation-sensitive focus detector, the detector comprising two detectorelements separated by a strip and the lens system having a pupil whichis larger than the cross-section of the reflected focus detection beamand being arranged so as to deflect this beam across over the focusdetector if the printing medium shifts from the focal plane of theimaging system.
 10. A module as claimed in claim 4, characterized inthat the focus error detection system comprises a radiation source forsupplying a focus detection beam, a focusing lens and a beam splittersuccessively arranged an the beam path between the radiation source andthe plane of the printing medium, and an astigmatic element and aradiation sensitive focus detector successively arranged in the path ofthe focus detection beam reflected from the printing medium, the focusdetector comprising four detector elements arranged in four differentquadrants of an x-y coordinate system in the plane of the detector. 11.A module as claimed in claim 4, characterized in that the focus errordetection system comprises a portion of the radiation-sensitivedetection system and an electronic processing which is circuit coupledto said portion to process and analyze the signals from the detectorelements of said portion in order to determine for which position of theaxially movable element the image of a feature of the medium has maximumcontrast and/or minimum size.
 12. A module as claimed in claim 1,characterized in that the radiation-sensitive detection system comprisesa colour beam splitter for splitting radiation incident thereon intoradiation of different colours, and further comprises a correspondingnumber of different kinds of detector elements, each kind beingsensitive to radiation of a different one of said colours.
 13. A moduleas claimed in claim 1, characterized in that a separate colour filter isarranged, in front of each detector element of the radiation-sensitivedetection system the colour filters for neighboring detector elementstransmitting radiation of different colours.
 14. A module as claimed inclaim 1, characterized in that the illumination system comprises atleast one set of different light-emitting diodes each emitting radiationof a different colour.
 15. A module as claimed in claim 1, characterizedin that the illumination system comprises a convergent lens system. 16.A module as claimed in claim 1, characterized in that the illuminationsystem comprises a cylindrical lens to form an elongated illuminationspot on the medium, the longitudinal direction of the spot beingperpendicular to the direction of a print line.
 17. A dot printer forwriting an image on a recording medium by exposing the medium on adot-by-dot basis and comprising: a communication channel for carrying aninformation signal representing the image to be printed at least onewriting device coupled to the communication channel and modulated by theinformation signal for producing dots on the recording medium, togethercomposing a printed image; a carriage for carrying the writing devicefor producing a relative movement between the recording medium and thewriting device, and a calibration instrument, fixed to the carriage, toread the printed dots and adjust printer settings, characterized in thatthe calibration instrument comprises a module as claimed in claim 1.